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SubjectRe: [RFC, 2.6] a simple FIFO implementation

> I did a similar test once for ppc that found that an increment followed by
> a test (marked unlikely) was actually quicker in practice than modulo
> arithmetic. The branch predictor got it right most of the time, so the
> test was almost free.

Yep x % y where y isnt constant could result in a divide which will blow
away 60+ cycles on some ppc machines. You can do a whole lot in those 60
cycles. Many memcpys for example :)

We removed all modulo arithmetic in the e1000 driver (replaced it with
if (x > y) x = 0) and managed to measure an improvement.

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